1. Technical Field of the Invention
The present invention relates to a demodulator and, more particularly, to a demodulator for performing buffer-less de-skewing for symbol combination.
2. Description of Related Art
Wireless communication has become an essential necessity of people's everyday life. Having access to all the communication features people have been accustomed to while “on the move” has been desired. Voice communication and data communication need to be available to people through their portable devices in a cost efficient way while maintaining acceptable quality.
Code Division Multiple Access (CDMA) wireless communication is a well adopted communication standard that is used throughout the world. In a CDMA wireless communication system, a user signal is spread with a wide frequency bandwidth by the use of an individual code and is transmitted in a common frequency band. The receiver detects a desired signal by a despreading process from the CDMA signal and the individual code.
The receiver in the CDMA wireless communication system often detects multiple signals from one or more base stations and demodulates these signals in the mobile station. As illustrated in FIG. 1, the propagation of the received signals from the base stations 12 is affected by the terrain (14 and 16) and may occur over multiple paths. These multiple path signals received from the same originating transmitted signal are referred to as multipath propagation signals.
This phenomenon of multipath propagation is due to various terrain factors such as scattering, refraction and reflection. Each multipath propagation signal requires a different amount of time to reach the receiver (mobile station 10). For example, a signal 18(1) received directly from a transmitter “line of sight” will be received before the reflected propagation signal 18(2). Nonetheless, each of the multipath propagation signals 18(1) and 18(2) is utilized by the receiver to provide a better signal quality by combining the multipath propagation signals together to form a stronger signal. Because the received multipath propagation signals are shifted in time with respect to each other, a de-skewing operation is necessary to align the signals and enable the signal combination.
FIG. 2 illustrates a conventional demodulator 50 in a CDMA wireless communication system. Signals are received on different physical channels (22, 24, and 26). The signals from each channel, however, propagate through different propagation paths (for example, path #1 (20), path #2 (30), and path #3 (40)). Symbols 28, 38, and 48 within each of the received signal channels 22, 24, and 26, respectively, are de-rotated and placed in a buffer/memory 54. Thus, the symbols from each physical channel and from each propagation path are stored in the buffer 54. The symbols 28 from a first channel 22 are received from propagation path 20 and are stored in the buffer 54 at location 56. The symbols 28 from the same first channel 22 received from the propagation path 30 are stored in the buffer 54 at location 58. The symbols 28 from the first channel 22 are received from propagation path 40 and are stored in the buffer 54 at location 60. Although only three paths are shown, it will be understood that the receiver is capable of handling more or fewer paths per physical channel.
The same operation is performed for a second channel 24 for all the propagation paths, i.e., propagation paths 20, 30, and 40, with the symbols 38 for the second channel 24 stored in the buffer 54 at locations 62, 64, and 66. The same operation is also performed for the rest of the channels, i.e., channel (C) 26, for all the propagation paths, i.e., propagation paths 20, 30, and 40, with the symbols 48 for that channel (C) 26 stored in the buffer 54 at different locations. Thus, each symbol received on each physical channel from each propagation path is stored in a distinct buffer location.
This buffering process allows the demodulator 50 to wait for the symbols (28, 38, and 48) to be received from each path before processing the symbols. De-skewing is automatically performed when the last symbol of a propagation path is aligned in the buffer 54. The symbols of a physical channel received from different propagation paths are then added using an adder 74 and output to one of the accumulators (76, 78, 80) associated with that physical channel.
The process for waiting for the symbols from all the propagation paths of the physical channel to be received before combining these symbols produces a delay in the demodulator output. The delay required to perform the symbol combination is at least the maximum path delay of the received signals. Moreover, storing the received symbols in a buffer before the symbol combination increases the power consumption of the demodulator. In view of the foregoing, there is a need for a demodulator that decreases the delay and reduces the power consumption of the demodulator.